1. Field of the Invention
The invention relates to an arrangement with a plurality of peripheral units and a sensor, where each of the peripheral units includes a connection for connecting the sensor to a supply voltage, includes a sensor input for connecting the sensor and also includes a measuring resistor for acquiring a sensor current that represents a signal state, and relates to a peripheral unit according which is suited to such an arrangement.
2. Description of the Prior Art
In the field of automation, high-availability solutions (H-systems), which are suited to reducing possible idling times of the plant to a minimum, are increasingly in demand. These H-systems conventionally have two automation devices that have a number of modules or programmable logic controllers, where both automation devices can access the peripheral units or decentralized peripherals connected to the automation devices and provided with a number of peripheral modules in read and/or write mode. To ensure that both automation devices can run in synchrony, these are synchronized at regular intervals.
Furthermore, it is frequently also desired in high-availability systems to redundantly operate two peripheral modules of a decentralized peripheral or two peripheral modules of two such decentralized peripherals. In the case that these peripheral modules are formed as digital input modules, an (individual) sensor is typically connected to both (partner) peripheral modules, where the decentralized peripherals are connected via a bus to an automation device, which has a number of modules, in the manner of a central or expansion device. However, in the case that the sensor is formed as what is known as a NAMUR sensor according to DIN 19234 that is widely used in process automation, it is not readily possible to operate this (individual) NAMUR sensor on two digital input modules simultaneously, because in this case inconsistent signal states would result on account of the impressed current supplied by the NAMUR sensor.
For instance, a peripheral unit formed as a digital input module and known from the Siemens handbook “Dezentrale Peripherie ET 200S, Digitales Elektronikmodul 4DI NAMUR” [Decentralized Peripheral System ET200S, Digital electronics module 4DI NAMUR], Edition March 2010, applies a supply voltage of 8.2 V to a connected NAMUR sensor, where different signal states “0”, “1”, “wire break” or “short-circuit” are mapped via currents impressed by the sensor. A current of less than 0.35 mA represents a wire break between the NAMUR sensor and digital input module, a current of 0.35 mA to 1.2 mA a signal state “0”, a current of 2.1 mA to 7 mA a signal state “1”, and a current of greater than 7 mA a short-circuited NAMUR sensor. The current flows via a low-impedance measuring resistor (shunt) of the digital input module and at this measuring resistor (1 kOhm) effects a voltage drop that is proportional to this impressed current, which maps or represents the signal state of the NAMUR sensor within the module.
In the case that the NAMUR sensor of the digital input module impresses a current of 2.4 mA, for instance, a voltage of 2.4 V drops at the measuring resistor, where this voltage drop is interpreted by the digital input module as signal state “1”.
Conversely, in the case that the NAMUR sensor is connected simultaneously to two such digital input modules and operated, i.e., the sensor is connected to an input of each of these digital input modules, the measuring resistors of the modules are connected in parallel. As a result, the current of 2.4 mA impressed by the NAMUR sensor is divided proportionally, for instance, and therefore only drops 1.2 V at each of the two measuring resistors of the two modules. This voltage drop of 1.2 V is assessed by the digital input modules as signal state “0”, which means that an incorrect signal state was “detected”.
In order to be able to form a clear signal state during redundant operation, it is therefore absolutely imperative that each of the two digital input modules has to know the availability of its partner module at any point in time in order to correctly assess the acquired voltage. In order to effect this, one NAMUR sensor can therefore be provided for each digital input module or digital input unit, in order to ensure the high availability of NAMUR sensors.